Method for making hollow plastic rotational casting and for removing casting from mold



3,214,506 STING Oct. 26, 1965 G. T. CORBIN, JR

METHOD FOR MAKING HOLLOW PLASTIC ROTATIONAL CA AND FOR REMOVING CASTING FROM MOLD Filed 001:. 1. 1962 2 Sheets-Sheet 1 INVE TOR. GEORGE T CORBIN JR. fiwmw Oct. 26, 1965 G. "r. CORBIN, JR

METHOD FOR MAKING HOLLOW PLASTIC ROTATIONAL CASTING AND FOR REMOVING CASTING FROM MOLD 2 Sheets-Sheet 2 Filed Oct. 1, 1962 \Q Q b Q INVENTOR. GEORGE T CORBIN JR.

ATTYS.

United States Patent METHUD FOR MAKING HOLLOW PLASTIC ROTA- TEONAL CASTING AND FOR REMOVING CAST- llNG FROM MOLD George T. Corbin, In, 421 Byberry Road, ll-liuntiugdon Valley, Pa. Filed Oct. 1, 1962, Ser. No. 227,330 2 Claims. (Cl. 264-310) This invention relates to method for making a hollow plastic casting or molding and has for an object the provision of improvements in this art.

One of the particular objects of the invention is to produce a hollow casting which has a very uniform wall thickness throughout all critical areas.

Another object is to produce a hollow casting having reduced ends and uniform wall thickness.

It may be helpful to describe the environment of one end use for which the invention has especially usefulness. This environment is in the field of jet-propelled objects or rockets. The directional jets of rockets require a closed container with a jet nozzle for holding the propellant charge such as solid fuel.

In order to have maximum strength to weight and volume these containers are commonly made of spun glass fiber and plastic wound on a mandrel, the layers being wound in alternation in oblique and straight circumferential directions when a portion of the container is cylindrical and the ends closed, as hemispherical for example.

It is highly important that these rocket charge containers be formed to very accurate dimensions and this requires that the mandrels on which they are wound have very precise dimensions and uniform wall thickness and strength throughout to resist the winding tension.

Since the wound container has only a small opening in one end there is a serious problem in removing the mandrel after the container has been formed and hardened on it. One practice is to make the mandrel of frangible material and break it into small pieces for removal through the end nozzle hole but this is very laborious and time-consuming task. The amount or material to be removed can be reduced by making the mandrel hollow and this is now the common practice.

Recently a composition material has been produced which is well suited for making hollow mandrels. It is referred to as a plastic but it appears to be crystalline and electrolytic and has a latent heat of melting and solidifying like water and, like water, portions solidify in the presence of residual liquid, like ice forming in water. One form of such material is marketed under the trade name Paraplast, by Rezolin Inc. of Santa Monica, California. This material is very hard like cement when solid and has the necessary rigidity and strength to make very satisfactory winding mandrels. It is also soluble in water and this makes its removal easy after the container has been formed. The mandrel is simply dissolved in water and drained out of the container, this being done without in any way damaging the inside surface of the container as is likely to occur when the mandrel must be broken or scraped out of the container. This plastic material, of a type suitable for making hollow cast mandrels, melts at about 400 degrees F. to 500 degrees F. and can be kept in a clean melted form ready for use in a closed electric resistance melting furnace.

However, the fact that the material solidifies in the presence of liquid portions requires that from the very beginning of solidification the material must be evenly distributed or washed over the surfaces of the mold in which it is solidified, with no appreciable time allowed for the start of solidification before the liquid circulation 3,2l4,5% Patented Get. 26, 1965 is started. It also requires that the mold walls be very uniform in thickness and Without local thick and thin zones over all critical areas; also that the apparatus which is used for turning the mold is free from local metallic contact with the mold, all support or securement elements being of heat insulating material. Further, the mold should be at the same temperature as the material at the time it is poured into the mold.

As a practical matter, this means that the mold, which must be made of assembled parts to permit removal of the casting, should be assembled, heated, and mounted in the turning apparatus before the material is poured into it so that turning can be started immediately after pouring without waiting for parts of the mold to be assembled, closed or mounted.

Another consideration of importance is that gases need to be vented from the mold during solidification of material in the mold. Heretofore it has been the practice to cast in successive stages or to break through the material at some point after solidification to vent the gases.

According to the present invention the necessary factors for certainly and quickly making good hollow castings are provided by a combination of suitable apparatus and procedure of an improved nature.

The improved procedure will best be understood from a description of an exemplary embodiment, reference being made to the accompanying drawings, wherein:

FIG. 1 is a medial axial section through a mold embodying the invention;

FIG. 2 is a right end elevation, partly in section, of the mold shown in FIG. 1;

FIG. 3 is a side elevation of a mold being assembled for use;

FIG. 4 is a diagrammatic section and elevation of a mold in a preheating furnace;

FIG. 5 is a diagrammatic view of a mold secured in a cage of a gyratory turning device and molten material being poured into the mold from a melting furnace;

FIG. 6 is a sectional diagrammatic view showing how molten material is held in the mold without spilling when the interior fill and vent spout is in its lowest position;

FIG. 7 is a sectional diagrammatic view showing how molten material is held in the mold when the fill and vent spout is disposed horizontally;

FIG. 8 is a diagrammatic view showing the mold being turned in the gyroscopic or gimbal machine to form an even thickness of material as it hardens;

FIG. 9 is a partial section of the drive end of the mandrel after the dummy pole piece and mold end cap at that end have been removed;

FIG. 10 is a transverse section taken on the line 1010 of FIG. 9;

FIG. 11 is a sectional view showing how the mandrel is removed from the cylindrical part of the mold; and

FIG. 12 is a central axial section of a finished mandrel.

As shown in FIG. 1, the mold, as of aluminum, is generally designated by the numeral 20 and includes a cylindrical body section 21, a bottom cap section 22 of semispherical shape, and a top or fill cap section 23 of semispherical shape. The sections are secured together at interfitting joints which are of the same thickness as the remainder of the mold Wall, each joint including an inner concentric rib 24 on one section fitting within an annular recess 25 of the adjacent section and an outer concentric rib 26 on the second section fitting within an annular recess 27 in the first section. The inner rib or projection 24 is radially thicker than the outer rib or projection 26 and its end surface engages the end surface of the recess 25; whereas the end surface of the outer projection 26 is spaced slightly away from the end wall of the recess 27. This provides that when the mold sections are forced to- 3 gether, as by clamping means, the inside surface of the mold will be fluid tight.

Drive positioning pins 30, two for each joint, properly locate the joint parts, axially and circumferentially, and cap bolts 31 secure the parts together. No holes extend through to the inside.

The mold is completed by a bottom or drive end dummy pole piece 34 and a top or fill end dummy pole piece 35. The pole pieces are located by positioning pins 36 and are secured to the mold caps by cap screws 37, both pins and screws extending through a flange 38. Threaded holes 39 are provided in the flanges 38 of the pole pieces to take reaction cap screws 39a for forcing the pole pieces away from the mold caps (by Wheel-puller action) after the cap screws 37 have been removed when disassembling the parts.

At the bottom end a dummy drive pin 40 is carried on the inner side of the dummy pole piece 34 for making a hole in the casting to take an active drive pin of an active pole piece which is used to turn the cast mandrel when the shell is later wound on it. A dummy center core plug 41 is also carried on the inner side of the dummy pole piece 34 to center the plug of the active drive pole piece when later secured on the cast mandrel.

At the top or fill end a fill tube or spout 43 is carried on the inner side of the dummy pole piece 35, the pole piece having a hole 44 forming a continuation of the bore of the tube. The tube may either be made integral with the pole piece or screwed into a threaded hole therein. The tube is tapered on the outer surface for easy removal from the material which hardens around it.

In FIG. 1 the approximate inner outline of the cast mandrel M which is to be formed is indicated by broken lines.

In FIG. 3 the parts 21, 22 and 23 are shown as being assembled to form the full mold 20, the dummy pole pieces 34 and 35 having been assembled on the caps 22 and 23. The fill spout 43 is shown on the top pole piece.

In FIG. 4 the assembled mold 20 is shown in an electric resistance muffie heating chamber 50 where it is brought up to a temperature somewhat above that of the material to be poured into it so as to be at about the pouring temperature after being taken out of the heating chamber. The pouring temperature may be about 500 degrees F., so the mold is heated to about 530 degrees F. to allow for cooling before use. Care is taken to heat the mold evenly around its circumference and from end-toend, as by a plurality of circumferentially spaced axially disposed electrical heating elements spaced at a short distance from the mold and controlled by a spirally disposed thermostatic element of known type.

After the mold has been heated it is secured in a cage 51 of a gyroscopic or gimbal turning device of known type having turning movement about two coordinate axes both passing approximately through the center of volume of the interior of the mold. This is shown in FIG. 5. Care is taken that all parts of the mold which engage holding means of the cage are held by heat insulating elements 52, on the sides, and 53, top and bottom. The top 54 of the cage may be provided with hinges 55 and a quick-lock means 56 for securing the mold tightly in position in a minimum time.

The top of the mold is open for pouring material into its spout hole and the body of the mold is almost completely exposed to the air for even cooling.

The mold holding cage 51 is mounted on a gyroscopic or gimbal device 60 having casters 61 for ready movement about on the floor. It may be taken to a material melting furnace 63 where the material is kept at a fixed temperature and which is provided with means for pouring a predetermined amount of material as required for a casting. By this arrangement it is not necessary to pour part of the material back out of the mold after a part of desired thickness has been formed as has been a prior practice, at least to some extent, with the type of material herein used.

The .mold is checked by a test thermostat and when it has cooled to the temperature of the material in the furnace a measured amount of molten material is poured into it from the furnace through a trough 64. While not shown in detail, it is understood that there is a liquid flow control device or gate 65 on the furnace above the trough 64.

It is here to be noted from FIGS. 6 and 7 that the interior fill and vent spout 43 of the mold is a carefully designed and very important part of the mold assembly. It has a bore opening of a diameter large enough to provide easy and rapid pouring of molten liquid into the mold and yet of a diameter small enough to avoid the outflow of liquid when the mold is turned on the side, as shown in FIG. 7. This must be maintained even if the tubular middle section of the mold is omitted and the two end caps are secured directly together (as the joints provide, as may be observed from FIG. 1) to form a spherical hollow casting instead of a spherical casting with semi-cylindrical ends, as shown.

Also, as shown in FIG. 6, the fill spout must be long enough to prevent liquid from flowing out when the mold is turned to a position with the tube end downward, yet short enough to avoid contact with liquid material when the mold spout is in the top position, even for a simple spherical mold.

With this arrangement gases are vented at all times while the mold is being turned about two axes and the material is hardening as the mold cools, yet no liquid can spill out an any time or get into the spout to hinder the venting of gases.

As indicated in FIG. 8, the mold is turned until the material all hardens. To aid this the air may be caused to flow past the mold, as by a fan.

After the material has all fully hardened in the mold and has had time to obtain good strength the mold is removed from the cast mandrel. The present mold has been especially designed to permit easy removal without injuring the cast article.

First, the cap screws 37 of the dummy pole piece 34 are removed and the pull-off cap screws 39a are turned in to press their ends against the end surface of the bottom end cap 22 thus forcing the dummy pole piece out of the cap 22 and casting. The drive pin 40 and core plug 41 may either come off with the pole piece or be left in the casting. If left in the casting they may easily be removed by tapping on them to loosen them from the casting and then pulling them out.

Next, the shell portion M1 of the mandrel which has formed over the end of the core plug 41 is knocked out. Its removal is indicated in FIG. 9.

A rod is then inserted in the hole thus made in the bottom end of the casting and the spout 43 is rapped on the sides by the rod to break up the thin coating of hardened material M2 which has formed on it.

The cap screws 37 of the top dummy pole piece 35 are removed and the pull-off screws 39a turned in, as for the bottom dummy pole piece, and the top pole piece with its pouring spout 43 removed.

Next the cap screws 31 and the pins 30 for the joints are removed and the caps 22 and 23 are removed from the casting. The bottom cap could be left on until the casting has been removed, if desired, since part of the other end is exposed for engagement for pushing the casting out of the mold.

Finally, as shown in FIG. 11, the mold is wrapped on the outside by a layer of heat insulating material 70, as a glass wool mat, and air is blown through the interior of the casting, as by a blower 71, to cause the casting to shrink in the cylindrical mold part 21. By a light rapping on the mold and an end push on the casting it is readily pushed out of the mold.

The finished casting is shown in FIG. 12. Since it is hygroscopic it is wrapped in a moisture excluding covering, as a sealed polyethylene bag, before shipment. It is usual also to secure pole pieces in the ends to assist in handling.

It is seen that the casting has very uniform wall thickness throughout the entire outer surface on which the subsequent windings will exert radial pressure, only the ends at the pole pieces being of non-uniform thickness but here it does not matter.

It is thus seen that the invention provides an improved method for making uniformly better mandrel castings than have heretofore been attainable.

While one embodiment of the invention has been specifically disclosed for purposes of illustration it is to be understood that there may be various embodiments and modifications within the general scope of the invention.

I claim:

1. The method of making a hollow casting with generally hemispherical shaft-supporting ends and uniform wall thickness over all outer curved zones from a watersoluble heat-meltable material which hardens at normal temperatures, as for a winding mandrel, which comprises:

providing a metal mold with a cylindrical mid-portion and generally hemispherical end caps having end openings and equatorial transverse joints with the cylindrical mid-portion, the mold having walls of uniform thickness, including the joints, over all outer curved areas,

providing bolted-on pole pieces closing the ends of the caps, the bottom end pole piece having a hole-defining core plug and the top end pole piece having an interior fill and vent spout of a diameter to permit quick filling with liquid material but preventing outflow of liquid when the mold is turned on the side and having a length such that liquid cannot flow out when the spout is disposed lowermost but short enough to prevent liquid from engaging the spout when it is disposed uppermost,

assembling the mold parts ready to receive molten liquid material, heating the assembled mold evenly in a closed mufile heater up to a temperature somewhat above the temperature of the molten liquid to be poured into it,

melting the material in a closed furnace under protected conditions, securing the heated mold with the fill spout end uppermost in a cage of a gyratory gimbal turning machine with heat insulation against all metal contact with the mold and with most of the mold exposed to the air, adjusting the mold to be at approximately the same temperature as the molten liquid in the furnace,

pouring from the furnace into the mold through its spout the correct amount of molten liquid to be formed into the hollow casting,

cooling the mold to solidify said material in a coating on the interior walls of the mold,

leaving the fill and vent spout open for venting while the material solidifies, immediately after pouring beginning the turning of the cage to flow the liquid evenly over the interior mold surfaces, and continuing the turning movement until all of the material hardens to form a hollow casting in the mold, removing the mold from the cage, removing the bottom end pole piece and breaking a hole through material which formed on the end of the core plug,

inserting a rod through the hole in the bottom end and rapping the fill spout to break off the material which hardened thereon,

unfastening the bottom end cap and removing it from the hollow casting,

unfastening and pulling off the top end pole piece,

unfastening the top end cap and removing it from the hollow casting,

covering the mold exteriorly with heat insulating material and blowing air through the interior of the hollow casting to shrink its diameter,

and removing the hollow casting from the mold by relative endwise movement between the hollow casting and the mold.

2. The method of making a hollow casting with generally spherical-shaped ends and uniform Wall thickness from a water-soluble heat-meltable material which hardens at normal temperatures, as for a winding mandrel, which comprises:

providing a separable metal mold having generally spherical-shaped end caps with axial openings therein, said mold having circumferential joints and walls of uniform thickness including the joints,

providing pole pieces fixed in said openings in the caps,

one of said pole pieces having a hole-defining core plug and the other having an interiorly-extending fill and vent spout of a diameter to permit quick filling with liquid material but preventing outflow of liquid when the mold is turned on the side and having a length such that liquid cannot flow out when the spout is disposed lowermost but short enough to prevent liquid from engaging the spout when it is disposed uppermost,

assembling the mold parts ready to receive molten liquid material, heating the assembled mold evenly up to a temperature somewhat above the temperature of the molten liquid to be poured into it, melting the material and heating it evenly to a predetermined temperature outside of the mold,

securing the heated mold with the fill and vent spout end upper most in a cage of a gyratory gimbal turning machine with heat insulation against all substantial metal contact with the mold and with most of the mold exposed to the air,

adjusting the mold to be at approximately the same temperature as the molten liquid,

pouring into the mold through its spout the correct amount of molten liquid to be formed into the hollow casting,

cooling the mold to solidify said material in a coating on the interior walls of the mold,

leaving the fill and vent spout open for venting while the material solidifies,

immediately after pouring, turning the cage to flow the liquid evenly over the interior mold surfaces, and

continuing the turning movement until all of the material hardens to form a hollow casting in the mold, removing the mold from the cage,

removing said one pole piece and forming a hole through the hardened material which formed on the end of the core plug,

inserting a rod by way of said formed hole and moving said rod to rap and break away the material hardened on the fill and vent spout,

covering the mold exteriorly with a heat insulating material and blowing air though the interior of the hollow casting to shrink its diameter, and

removing the hollow casting from the mold.

ROBERT F. WHITE, Primary Examiner.

ALEXANDER H. BRODMERKEL, Examiner. 

1. THE METHOD OF MAKING A HOLLOW CASTING WITH GENERALLY HEMISPHERICAL SHAFT-SUPPORTING ENDS AND UNIFORM WALL THICKNESS OVER ALL OUTER CURVED ZONES FROM A WATERSOLUBLE HEAT-MELTABLE MATERIAL WHICH HARDENS AT NORMAL TEMPERATURES, AS FOR A WINDING MANDREL, WHICH COMPRISES: PROVIDING A METAL MOLD WITH A CYLINDRICAL MID-PORTION AND GENERALLY HEMISPHERICAL END CAPS HAVING END OPENINGS AND EQUATORIAL TRANSVERSE JOINTS WITH THE CYLINDRICAL MID-PORTION, THE MOLD HAVING WALLS OF UNIFORM THICKNESS, INCLUDING THE JOINTS, OVER ALL OUTER CURVED AREAS, PROVIDING BOLTED-ON POLE PIECES CLOSING THE ENDS OF THE CAPS, THE BOTTOM END POLE PIECE HAVING A HOLE-DEFINING CORE PLUG AND THE TOP END POLE PIECE HAVING AN INTERIOR FILL AND VENT SPOUT OF A DIAMETER TO PERMIT QUICK FILLING WITH LIQUID MATERIAL BUT PERVENTING OUTFLOW OF LIQUID WHEN THE MOLD IS TURNED ON THE SIDE AND HAVING A LENGTH SUCH THAT LIQUID CANNOT FLOW OUT WHEN THE SPOUT IS DISPOSED LOWERMOST BUT SHORT ENOUGH TO PREVENT LIQUID FROM ENGAGING THE SPOUT WHEN IT IS DISPOSED UPPERMOST, ASSEMBLING THE MOLD PARTS READY TO RECEIVE MOLTEN LIQUID MATERIAL, HEATING THE ASSEMBLED MOLD EVENLY IN A CLOSED MUFFLE HEATER UP TO A TEMPERATURE SOMEWHAT ABOVE THE TEMPERATURE OF THE MOLTEN LIQUID TO BE POURED INTO IT, MELTING THE MATERIAL IN A CLOSED FURNACE UNDER PROTECTED CONDITIONS, SECURING THE HEATED MOLD WITH THE FILL SPOUT END UPPERMOST IN A CAGE OF A GYRATORY GIMBAL TURNING MACHINE WITH HEAT INSULATION AGAINST ALL METAL CONTACT WITH THE MOLD AND WITH MOST OF THE MOLD EXPOSED TO THE AIR, ADJUSTING THE MOLD TO BE AT APPROXIMATELY THE SAME TEMPERATURE AS THE MOLTEN LIQUID IN THE FURNACE, POURING FROM THE FURNACE INTO THE MOLD THROUGH ITS SPOUT THE CORRECT AMOUNT OF MOLTEN LIQUID TO BE FORMED INTO THE HOLLOW CASTING, COOLING THE MOLD TO SOLIDIFY SAID MATERIAL IN A COATING ON THE INTERIOR WALLS OF THE MOLD, LEAVING THE FILL AND VENT SPOUT OPEN FOR VENTING WHILE THE MATERIAL SOLIDIFIES, IMMEDIATELY AFTER POURING BEGINNING THE TURNING OF THE CAGE TO FLOW THE LIQUID EVENLY OVER THE INTERIOR MOLD SURFACES, AND CONTINUING THE TURNING MOVEMENT UNTIL ALL OF THE MATERIAL HARDENS TO FORM A HOLLOW CASTING IN THE MOLD, REMOVING THE MOLD FROM THE CAGE, REMOVING THE BOTTOM END POLE PIECE AND BREAKING A HOLE THROUGH MATERIAL WHICH FORMED ON THE END OF THE CORE PLUG, INSERTING A ROD THROUGH THE HOLE IN THE BOTTOM END AND RAPPING THE FILL SPOUT TO BREAK OFF THE MATERIAL WHICH HARDENED THEREON, UNFASTENING THE BOTTOM END CAP AND REMOVING IT FROM THE HOLLOW CASTING, UNFASTENING AND PULLING OFF THE TOP END POLE PIECE, UNFASTENING THE TOP END CAP AND REMOVING IT FROM THE HOLLOW CASTING, COVERING THE MOLD EXTERIORLY WITH HEAT INSULATING MATERIAL AND BLOWING AIR THROUGH THE INTERIOR OF THE HOLLOW CASTING TO SHRINK ITS DIAMETER, AND REMOVING THE HOLLOW CASTING FROM THE MOLD BY RELATIVE ENDWISE MOVEMENT BETWEEN THE HOLLOW CASTING AND THE MOLD. 